A variety of techniques are presently in use to enable chemical analysis of ions in ion mixtures. Many such techniques rely on introducing in a gaseous atmosphere, ions from a substance originally dissolved in a liquid, and then analyzing such ions in a mass spectrometer. Sometimes, however, a large number of different species produced by the ionization process give rise to too complex a mass spectrum for individual mass peaks to be recognizable. This circumstance is particularly common for ion sources such as electrosprays, which yield each species in a multitude of charge states, especially in the case of analytes with very large molecular weights.
An electrospray atomizer is implemented by feeding an electrically conductive liquid through a tube, while a liquid meniscus emerging out of the tube is maintained at several kilovolts relative to a reference electrode positioned a few tube diameters away. This liquid meniscus assumes a conical shape under the action of the applied electric field, with a thin jet of solvent/solute emerging from the cone tip. This jet breaks up farther down stream into a spray of fine, charged droplets.
A distinct advantage which arises from the electrospray apparatus is that the generated droplets exhibit a net charge on their respective surfaces. This charge enables the particles to be guided and collected for a variety of purposes. Further, charge repulsion among the droplets prevents an agglomeration thereof. As liquid in the charged droplets evaporates, the electric charge may cause the droplets to further subdivide, which eventually creates arbitrarily small droplets, residue particles and even ions. Many of such particles hold enough charge not to be readily separated from the ions, giving rise to background noise in the mass spectrum.
Accordingly, a method for separating some of the ions in the gas from each other and from condensed particles in the electrospray is needed to simplify the mass spectra and also to reduce noise.
An efficient method for the separation of ions in a gas utilizes mobility analysis of ions and particles in the gas. Such analysis is based upon the differences in velocity at which different ions drift through the gas, in response to an applied electric field. One such method is based upon the time-of-flight of the ions/particles drifting between two points in the gas. If an analyzer is set only to be responsive to particles arriving within a preset time window, ion/particle selectivity can be achieved. Time-of-flight analysis has been successfully coupled with mass spectrometers to enable a determination of the mass of mobility-selected ions. However, in the time-of-flight analysis approach, ions are separated from each other in dependence upon a time of arrival at a detector. This complicates their introduction into most types of mass spectrometers which utilize atmospheric pressure sources. Such mass spectrometers tend to rely on the use of a steady, rather than a pulsed, ion input. Further, the approach of filtering out of the mass spectrometer inlet all ions from a time of flight mobility spectrum, except those within a small mobility range, causes a substantial loss in ion signal.
An alternative approach which avoids the low duty cycle that results from the use of the time-of-flight method, is based on mobility separation in space rather than in time. Instruments capable of performing a steady mobility selection of an ion stream are widely used in aerosol research, where they are commonly referred to as differential mobility analyzers (DMAs). Aerosol DMAs have traditionally been designed to isolate charged objects with relatively small mobilities. They exhibit rather poor resolution and very high losses for molecular ions of greatest analytical interest.
In a DMA, an electric field is established between two plane parallel metal plate electrodes (or two coaxial cylindrical electrodes) by a potential applied therebetween. Ions or particles are injected through a slit in a first electrode. As these charged entities drift in the electric field towards a second electrode, they are deflected in a direction parallel to the plate electrodes, by a flow of clean, ion-free gas which moves parallel to and between the surfaces of the plates. As a result, the position of the ions' impact on the second plate electrode, in relation to the gas stream direction, is an inverse measure of their electrical mobility. A further slit may be made in the second electrode, through which the charged species can be sampled. By alteration of the voltage, selected subsets of the ionic species can be caused to exit through the slit.
The response of DMAs is substantially influenced by Brownian motion which spreads particles of a given mobility around their mean trajectory. As a result, an aerosol sample through an exit slit contains a relatively wide range of ion/particles of varying mobilities (especially in the case of ultra-fine particles). Consequently, most existing aerosol DMAs do not function as monodisperse particle generators, nor do they measure mobilities of particles in the nanometer range with adequate resolution.
A further problem of aerosol DMAs relates to large losses of ions through the sampling lines which lead from an ion source to the analyzing section and then from the analyzing section to the instrument exit. Thus, most electrospray/mass-spectrometry devices sample ions into their vacuum systems at initial concentrations comparable to the very large ones prevailing in the electrospray source (or from an alternative ion source). However, the sampling process, in all available DMAs, reduces this concentration by more than the two orders of magnitude that are typically lost in coupling a time of flight mobility analyzer to a mass spectrometer.
Accordingly, it is an object of this invention to provide an apparatus and method for analysis of an ion stream in a gas, which apparatus and method exhibit a better transmission efficiency than time-of-flight instruments.
It is another object of this invention to provide an apparatus and method for analysis of an ion stream in a gas which enables exclusion of all but a narrow band of ions which exhibit a determined ion mobility.
It is another object of this invention to provide an apparatus and method for analysis of an ion stream in a gas which provides both precise ion selectivity, while providing a continuous stream of selected ions for analysis.
For complex mass spectra involving several large and multiply charged species, it is another object of this invention to provide a procedure to group together the peaks which correspond to same species present in different charge states. This grouping is equivalent to the identification of each of the species giving rise to the mass spectrum, which are otherwise difficult to resolve by mass spectrometry alone.